Photothermal-enabled single-atom catalysts for high-efficiency hydrogen peroxide photosynthesis from natural seawater

Hydrogen peroxide (H2O2) is a powerful industrial oxidant and potential carbon-neutral liquid energy carrier. Sunlight-driven synthesis of H2O2 from the most earth-abundant O2 and seawater is highly desirable. However, the solar-to-chemical efficiency of H2O2 synthesis in particulate photocatalysis systems is low. Here, we present a cooperative sunlight-driven photothermal-photocatalytic system based on cobalt single-atom supported on sulfur doped graphitic carbon nitride/reduced graphene oxide heterostructure (Co–CN@G) to boost H2O2 photosynthesis from natural seawater. By virtue of the photothermal effect and synergy between Co single atoms and the heterostructure, Co–CN@G enables a solar-to-chemical efficiency of more than 0.7% under simulated sunlight irradiation. Theoretical calculations verify that the single atoms combined with heterostructure significantly promote the charge separation, facilitate O2 absorption and reduce the energy barriers for O2 reduction and water oxidation, eventually boosting H2O2 photoproduction. The single-atom photothermal-photocatalytic materials may provide possibility of large-scale H2O2 production from inexhaustible seawater in a sustainable way.

where NHE represents the normal hydrogen electrode. Unfortunately, the sluggish water oxidation limits overall reaction kinetics of H 2 O 2 photosynthesis 11,12 . Consequently, additional carbonaceous sacrificial agents (e.g., methanol, isopropyl alcohol, and benzyl alcohol) need be added to match the half-reaction of O 2 reduction 13 , causing extra difficulty in the product separation. Especially, the pure water feed imposes a heavy burden on the scarce freshwater resource 14,15 . Alternatively, direct H 2 O 2 photosynthesis from inexhaustible seawater represents a more sustainable development pathway. Recent research on polymer-carbon dot composite and lignin-supported BiOBr photocatalysts showed that H 2 O 2 photosynthesis from seawater is receiving increasing attention 14,16 . Despite significant advances, such process in real seawater remains limited and is still in its infancy, especially the solar-to-chemical conversion (SCC) efficiency is far from the target for practical application and the stability of photocatalysts suffers from formidable challenges associated with the notorious salt chemistry. It is imperative to develop cost-effective and stable materials to improve the SCC efficiency of H 2 O 2 photosynthesis from natural seawater.
The steps toward H 2 O 2 production involve O 2 activation and subsequent protonation 17,18 . Thus, the absorption of O 2 and the relative energy levels of the reactive intermediates decides whether or how fast H 2 O 2 photosynthesis will actually happen. It is generally accepted that H 2 O 2 production could be realized via either a •O 2 − -mediated process or a direct O 2 reduction pathway 19,20 . Impressively, •O 2 − is typically generated in natural living organisms and can be effectively converted to H 2 O 2 via redox-active superoxide reductase/dismutase composed of transition metal cofactors and organic ligand moieties 21 . In this regard, heterogeneous single-atom catalysts, containing atomically dispersed metal active sites stabilized on a support with specific geometric and electronic structure, could be potentially considered as the mimics of natural metalloenzyme systems 22,23 . Moreover, the strong metal-support interaction would certainly tailor the electronic structure and induce the charge redistributions 24,25 , which could promote the adsorption of O 2 and eventually modulate the catalytic reactivity of photocatalysts. On this basis, it can thus be envisioned that, if we could anchor transition metal single atoms on sunlight-driven semiconductors to promote O 2 adsorption and activation, the resulting photocatalysts would greatly enhance H 2 O 2 production. However, incorporating transition-metal single-atom into photocatalytic semiconductors could induce serious carrier recombination due to the halffilled intermediate bands 26,27 . As such, transition metal single-atom photocatalysts need to be rationally designed to ensure the efficient use of photogenerated carriers before the materials become useful for H 2 O 2 photosynthesis.
Creating heterostructure or hybrid photocatalysts is considered as an essential strategy in the pursuit of advanced photocatalytic processes 28 . Considering the unique structure features and engineerability of two-dimensional (2D) materials, heterostructure photocatalysts based on 2D nanosheets provide considerable opportunities to induce intimate electronic coupling between semiconductor and substrate 29,30 , promoting the separation/migration of photogenerated carriers. In addition, broadening photo-response of photocatalysts and ensuring sufficient solar energy utilization are also crucial for improving the SCC efficiency. Notably, interfacial solar heating with a localized photothermal effect by converting low-energy photons into heat offers a promising pathway for incident light usage and becomes necessary for accelerating the reaction kinetics on the surface of adjacent semiconductors 31,32 . Several important chemical reactions were assisted via the corporation of photothermal effect and photocatalysis, such as CO 2 conversion 33 , hydrogen evolution reaction 34 , and nitrogen oxidation 35 . In some cases, the synergistic photothermalphotocatalytic process enhanced the stability of photocatalyst 34 . Thus, a direct integration of photothermal-photocatalytic heterostructure system may provide unprecedented possibilities for efficiently driving H 2 O 2 production under sunlight irradiation. So far, such a cooperative system remains less exploited in H 2 O 2 photosynthesis.
With the above in mind, we propose herein a sunlight-driven single-atom photothermal-photocatalytic system based on atomiclevel Co anchored on 2D sulfur doped graphitic carbon nitride/ reduced graphene oxide heterostructure (hereafter, referred as Co-CN@G) for non-sacrificial H 2 O 2 photosynthesis from natural seawater. The single-atom heterostructure can not only suppress the recombination of photogenerated carriers but also endow the photothermal effect to accelerate reaction kinetics. The optimized system achieves an impressive performance in H 2 O 2 photosynthesis from seawater, affording a high SCC efficiency of 0.72% under simulated sunlight illumination. DFT calculations confirm that atomic-level Co sites together with the intimate interlayer electronic coupling promote the charge separation/migration, O 2 reduction and water oxidation, ensuring the greatly enhanced photocatalytic performance in H 2 O 2 photosynthesis.

Photocatalyst synthesis and atomic structure
Co-CN@G nanohybrid was fabricated by in situ thermally polymerizing Co-thiourea complex absorbed on 2D reduced graphene oxide (RGO) sheet (Fig. 1a). The detailed synthetic procedure is given in "Methods". Co-thiourea complex formed by the coordination of cobalt ions with the nitrogen-rich and sulfur-containing thiourea precursor was first absorbed on the structure-directing RGO template. Further heat treatment at an optimized temperature of 400°C under an argon atmosphere promoted the formation of Co-CN@G nanohybrid. In this way, Co single atom coordinated on sulfur doped C 3 N 4 (CN) was immobilized on 2D RGO sheet from the polycondensation of Co-thiourea complex, providing sufficient contact area for interfacial electronic coupling. The structure of Co-CN@G was verified by X-ray diffraction (XRD) (Supplementary Fig. 1). A slight shift but stronger peak at 26.7°was observed in contrast with RGO because of the overlap of the (002) peak of CN and RGO, a solid testimony for the strong chemical coupling between Co-CN and RGO 36,37 . No additional diffraction peaks associated with Co were found, whereas the porosity and surface area increased based on the results of N 2 sorption analysis ( Supplementary Fig. 2). Fourier-transform infrared spectroscopy showed the typical tri-s-triazine, C-N and N-H stretching vibrations of graphitic CN ( Supplementary Fig. 3) 38 . Electron microscopy images indicated that Co-CN@G maintained the lamellar morphology without the formation of obvious particles (Supplementary Figs. 4 and 5). Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) confirmed the atomically dispersed Co single atoms (Fig. 1b). The STEM-coupled energydispersive spectroscopy element mapping revealed the homogeneous dispersion of atomic Co, C, N, and S over the whole sample (Fig. 1c). The sample gave a C/N atomic ratio of 2.75, as depicted in Supplementary Fig. 6. The high-resolution Co 2p XPS peaks located at 780.9 and 796.3 eV could be assigned to Co 2p 3/2 and Co 2p 1/2 , respectively (Supplementary Fig. 7) 39 . C 1s spectra in Co-CN@G hybrid displayed an extra peak at 287.3 eV assigned to the out-of-plane orientation of sp 3 C-N species (Fig. 1d), manifesting the interfacial chemical interconnectivity between Co-CN and RGO parallel layers 40 .
The atomic structure of Co-CN@G was examined. X-ray absorption near-edge structure spectra (XANES) of Co K-edge demonstrated the location of the absorption edge and the intensity of fingerprint peak (7730 eV) for Co-CN@G at the middle of the reference Co foil and Co 3 O 4 samples (Fig. 1e). These suggested the presence of single Co atoms in oxidized state 41,42 . Extended X-ray absorption fine structure (EXAFS) of Co-CN@G exhibited a dominant peak at~1.82 Å, which can be separated into two independent peaks assigned to Co-N and Co-S scattering. No metallic Co-Co peak at 2.1 Å could be seen, suggesting the existence of all Co as isolated atoms (Fig. 1f). The FT-EXAFS profiles ( Fig. 1g) and the corresponding fitting results (Supplementary Fig. 9 and Supplementary Table 1) verified that CoN 1 S 3 (one Co-N bond and three Co-S bonds) should be the dominating structure in Co-CN@G. Also, Co-CN@G displayed a strong wavelet transform maximum focused at 5.1 Å −1 assigned to Co-N/S contribution, which was sharply distinguished from the corresponding maxima of Co foil (Co-Co, 7.1 Å −1 ) and Co 3 O 4 (Co-O, 6.2 Å −1 ) ( Supplementary Fig. 10). 15 N solid-state nuclear magnetic resonance (NMR) measurement was employed to acquire the detailed information of N species at the atomic level 43,44 . The 15 N NMR spectra of Co-CN@G exhibited a poor signal-to-noise ratio ( Supplementary Fig. 11) due to the interference of conductive RGO and magnetic Co atoms. Alternatively, the pristine Co-CN was evaluated (Fig. 1h). Four typical resonance peaks at~135, 156,~107, and~191 ppm were observed, which could be assigned to the bridged NH, central NC 3 , NH 2 , and NC 2 in the tri-s-triazine ring, respectively 25,45 . Anchoring Co onto the CN support resulted in a decreased intensity of the 191-ppm NMR peak, suggesting that Co atoms bonded to the NC 2 sites on the CN surface.

Photoelectronic(thermal) property and electronic structure
Ultraviolet-visible diffuse reflectance spectroscopy indicated that Co-CN@G could cover the UV-visible-NIR light region (Fig. 2a). Compared with other samples, Co-CN@G showed an enhanced absorbance, implying that CN@G combined with Co single atom could utilize solar energy more effectively. Moreover, Co-CN@G exhibited a slight redshift in the optical absorption edge, corresponding to the distinct band gap shrinkage according to Kubelka-Munk function ( Supplementary Fig. 12). As revealed by valence-band XPS spectra ( Supplementary Fig. 13), the conduction band and valence band energy of Co-CN@G were ca. −0.62 and 2.0 eV (versus NHE), respectively. Notably, the narrowed bandgap and favorable energy band levels were enough for driving O 2 reduction and water oxidation thermodynamically. The emission peak centered at 436 nm in photoluminescence emission spectroscopy was markedly reduced on Co-CN@G despite its stronger photon absorption ( Supplementary  Fig. 14), implying that the radiative recombination was greatly suppressed 46 . The time-dependent photo-response curves at open-circuit voltage showed an elevated photocurrent than that of the other catalysts once exposed to the simulated sunlight irradiation (Fig. 2b), revealing that Co-CN@G had superior charge generation and separation capacity. The increased electrolyte temperature induced by photothermal effect could contribute to the gradual increase of photocurrent 47 . The facilitated charge carrier migration could be further confirmed by the decreased electrochemical impedance (Supplementary Fig. 15). We then explored the photothermal property of the samples by monitoring the time-dependent temperature variation of the suspensions under different photoirradiation ( Supplementary  Fig. 16). Upon continuous simulated sunlight illumination (AM 1.5 G, one sun) for 60 min, the system temperature spontaneously increased from 27.1 to 37.3°C for CN@G and 38.4°C for Co-CN@G, evidently higher than the resulting temperature of the blank solution or Co-CN suspension. By equipping with a 720 nm short-pass filter (NIR light was removed, labeled as UV-vis) or a long-pass filter (UV-vis light was removed, labeled as NIR), Co-CN@G suspension under UV-vis and NIR light exposure yielded a decreased temperature of 7.3 and 2°C respectively, verifying that the photothermal effect mainly originated from NIR light.
We then performed density functional theory (DFT) calculations to understand the interlayer electron transfer and electronic structure of the photocatalysts. The Mulliken charge difference between the adjacent layers in CN@G (~0.041 e − ) was larger than that in Co-CN (roughly 0.028 e − ), implying the strong charge transfer between the interlayers in CN@G (Supplementary Fig. 17) 5 . Obviously, the presence of Co-single-atom sites in CN@G drastically rearranged the charge distribution between the neighboring planes of Co-CN@G (Fig. 2c, d), resulting in an electron depletion in the fourth layer and a distinct electron accumulation on the first (0.021 e − ) and third layer (0.099 e − ). As a result, Co-CN@G exhibited a significantly larger charge difference between the adjacent layers (~0.124 e -). This result meant that the interlayer electronic coupling of Co-CN@G had been notably promoted by incorporating Co single atoms, facilitating charge separation and transfer. The electronic structures were further analyzed using density of states (DOS) ( Fig. 2e and Supplementary Fig. 18). The calculated d-band center of Co-CN@G (−0.97 eV) was much higher than that of Co-CN (−1.15 eV), indicating that Co-CN@G was more favorable to the binding and activation of O 2 molecule. Furthermore, the shape and height of peaks in the negative side changed because of the modification of Co 3d orbit. Atomically dispersed Co generated two distinct and occupied mid-gap states (~0.5 and 0.7 eV above the valence band maximum), decreasing the band gaps and promoting the localization of photoexcited holes 48 . Additionally, it was found from PDOS that the 3d xy orbital of Co atom strongly coupled with the 2p y / 2p z orbital of the N and S atom ( Supplementary Fig. 19), further confirming the strong coordination between Co, N, and S and the formation of in-plane bonds 49 .

Photocatalytic activity for H 2 O 2 production
Photocatalytic H 2 O 2 production on different catalysts was first evaluated in natural seawater under simulated solar irradiation (AM 1.5 G, 100 mW cm −2 ) without sacrificial reagents (Fig. 3a). No H 2 O 2 was detected in the dark or without photocatalyst, which verified the photo-induced reaction. Under light illumination, Co-CN@G heterostructure showed a significantly higher H 2 O 2 yield than Co-CN and CN@G, indicating that Co single atoms combined with CN@G contributed to the enhanced photocatalytic reactivity. By optimizing Co loading and mass ratio of components, Co-CN@G with 1.5 wt% Co and a CN/RGO mass ratio of 1:1 produced the largest amount of H 2 O 2 ( Supplementary Fig. 20). Excessive amounts of either Co, Co-CN, or RGO resulted in lower photocatalytic activity, which could be attributed to the formation of Co nanoclusters at high Co loading (Supplementary Fig. 21) or insufficient interfacial coupling between Co-CN and RGO. H 2 O 2 production on the optimized Co-CN@G dramatically increased with time at the early stage of irradiation (Fig. 3b). Notably, Co-CN@G generated 0.44 times higher H 2 O 2 amount in seawater (8.29 mM) than in pure water (5.77 mM) for the first 10 h. Also, Co-CN@G displayed a lower decomposition rate (K d ) of H 2 O 2 than other samples, and the temperature exerted a negligible effect on the decomposition behavior ( Supplementary Fig. 22).
Photosynthesis was further evaluated under different light irradiation to clarify the effect of incident light sources on H 2 O 2 production (Fig. 3c) illumination. In other words, NIR light irradiation played a positive effect on the photocatalytic reaction. In addition, the thermal-assisted photosynthesis under UV-vis light illumination was further examined. The H 2 O 2 amount increased as the reaction temperature elevated, revealing that the thermal effect progressively boosted the photocatalytic performance of Co-CN@G. Typically, H 2 O 2 evolved under UV-Vis light illumination at 42°C was just comparable to the amount under simulated sunlight irradiation, confirming that the localized surface heating through photothermal effect can promote the surface reaction kinetics. To further verify this, photosynthesis experiments were conducted by illuminating two different light sources simultaneously. A solar simulator (lamp 1) was placed on the one side of the vessel to offer the simulated sunlight illumination (100 mW cm −2 ). The lamp 2 with NIR light export was placed on the other side of the vessel. In this case, the effect of NIR light on the photocatalytic process can be evaluated by adjusting the power of lamp 2 (0, 50, 100 mW cm −2 ). As shown, Co-CN@G produced a higher level of H 2 O 2 with the increased NIR light intensities (Fig. 3d), while H 2 O 2 evolved on Co-CN was almost independent of the intensity of NIR light. Therefore, NIR light illumination exerted a vital but auxiliary role in strengthening the catalytic reactivity of Co-CN@G.
The SCC efficiency was determined to be 0.72% under simulated sunlight irradiation (Fig. 3e), which was about 19.8 and 2.5 times higher than that of pristine Co-CN and CN@G ( Supplementary Fig. 23), respectively, and superior to other particulate photocatalysts in pure water and natural seawater (Supplementary Table 2). However, the activity definitions used to evaluate catalysts for H 2 O 2 evolution are not standardized, imposing difficulties in comparing the intrinsic activity of different photocatalysts. The wavelength-dependent apparent quantum efficiency (AQE) of H 2 O 2 production was then evaluated under the monochromatic light illumination (Fig. 3f). Co-CN@G exhibited a maximum AQE of 9.1% at 420 nm. The photocatalytic stability was evaluated by collecting and reusing the photocatalyst for 15 cycles. Co-CN@G was stable enough to maintain its photocatalytic activity during repeated cycles (Fig. 3g). In contrast, the reference samples, especially Co-CN, yielded the gradually decreased H 2 O 2 amount after continuous cycling (Supplementary Fig. 24). The unchanged XRD pattern, Co 2p XPS spectra, FT-IR spectrum and inapparent morphology transformation further confirmed the robust nature of Co-CN@G (Supplementary Fig. 25). The Co content in the solution after continuous photocatalytic reaction for 120 h was below the detection limitation of inductively couple plasma mass spectrometer (ICP-MS) measurement, representing no Co leaching. We also performed the outdoor photocatalytic test on 24 July 2022, with an ambient temperature of 28-34°C and a sunlight intensity of 0.49-0.93 kW m −2 in the daytime in Haikou City of Hainan Province, China. Co-CN@G (30 mg in 50 mL seawater) achieved a daily total amount of H 2 O 2 of 5.1 mM (8508 μmol g −1 day −1 ) in the O 2 -saturated particulate suspension system under ambient sunlight irradiation, demonstrating the potential of large-scale H 2 O 2 production from seawater (Fig. 3h).
A series of experiments have been performed to elucidate the origin of performance enhancement in seawater. We subjected individual inorganic salt, NaCl, CaCl 2 , MgCl 2 , KCl, Na 2 SO 4 , and Na 2 CO 3 , to the freshwater (Supplementary Fig. 26). It was observed that the presence of these inorganic salts improved H 2 O 2 production. It has been reported that the electron sink of oxygen-containing groups of carbon materials in the presence of metal cations could boost photocatalytic activity by attracting electrons 14 . To explore the synthetic effect of seawater ions, simulated seawater with 3.4% salinity was used for further investigation since its composition is very close to those of natural seawater. As expected, the simulated seawater showed a similar H 2 O 2 formation with natural seawater (Fig. 3b). We observed a volcanic relationship between H 2 O 2 evolution and salinity with a summit at a moderate salinity of 2.4% ( Supplementary Fig. 27), illustrating that H 2 O 2 photosynthesis from natural seawater can be significantly enhanced by pretreatment of mild desalination. The photocatalytic performance was then examined when one cation was removed. To keep the same ion concentration, ammonium chloride or sulfate was introduced (Supplementary Table 3). We found that the photocatalytic activity was suppressed in the absence of Na + and Mg 2+ , while the removal of K + and Ca 2+ had little effect on H 2 O 2 formation because their actual concentrations in seawater are low (K + : 0.009 M, Ca 2+ : 0.01 M) (Fig. 3i). DFT calculations were performed to understand the effect of cation on the charge transfer ( Supplementary Fig. 28). Hydrated cations were considered to mimic the experimental environment during the calculations, with the optimized number of water molecules bound to Na + and Mg 2+ being three and four, respectively. Interestingly, the interlayer charge transfer of Co-CN@G was improved after interacting with Na + (H 2 O) 3 or Mg 2+ (H 2 O) 4 cation. This trend was also confirmed experimentally by resorting time-resolved PL spectroscopy ( Supplementary Fig. 29). Co-CN@G treated with different metal chlorides exhibited the increased intensity-average PL lifetime (τ), indicating that the presence of metal cations can improve the efficiency of photoexcited carriers. Next, we examined the reaction when halide ions were absent from simulated seawater ( Supplementary  Fig. 30). The H 2 O 2 evolution decreased due to the absence of Cl − in simulated seawater, indicating that Cl − exerted a positive effect on H 2 O 2 generation. The effect of low-concentration Br − (0.00087 M) can be neglected. A similar enhancement was also observed in a NaCl or NaBr solution. Although Cl − oxidation by photogenerated holes requires a higher potential than four-electron water oxidation, it is kinetically favorable owing to its two-electron process. The formed Cl 2 can further react with water to produce HClO, which is subsequently decomposed into Cl − and O 2 under light irradiation. During the photocatalytic process, Cl 2 was not monitored by gas chromatography. The net result is that Classists in the four-electron water oxidation. The O 2 amount generated in N 2 -saturated NaIO 3 aqueous solution with NaCl additive after 2 h (3.82 µmol) was 0.3 times higher than that with NaF additive (Supplementary Fig. 31), verifying that Cl − has an auxiliary effect on the water oxidation.

The photocatalytic mechanism for H 2 O 2 production
To understand the origin of H 2 O 2 evolution over Co-CN@G, a series of control experiments were performed. The generated intermediates during H 2 O 2 synthesis were explored using the electron paramagnetic resonance method with 5,5-diemthyl-1-pyrroline N-oxide (DMPO) as the spin-trapping agent. No DMPO-•O 2 − or DMPO−•OH signal was detected in the dark or under NIR light exposure (Fig. 4a). The signals of DMPO-•O 2 − and DMPO−•OH were obviously intense under UV-vis light and simulated sunlight irradiation, and the intensities under simulated solar illumination were higher than those under UV-vis illumination, revealing that the local heating effect generated by photothermal transduction of Co-CN@G motived the generation of radical intermediates. The addition of either NaIO 3 electron acceptor or benzoquinone (BQ) (•O 2 − scavenger) into the reaction system resulted in the sharply decreased H 2 O 2 , meaning that H 2 O 2 photosynthesis was dominated by O 2 reduction (Fig. 4b). Rotating disk electrode analysis conducted in an O 2 -saturated phosphate buffer solution (0.1 M, pH~7) indicated that the electron transfer number for O 2 reduction was~2 (Fig. 4c). Isotopic labeling experiments were further conducted. Co-CN@G photocatalyst in H 2 16 O and 18 O 2 mixture was illuminated for different times, and the resulting solution was then Co-incubated with MnO 2 to decompose H 2 O 2 into O 2 50 . The evolved gaseous product detected by gas chromatography-mass spectrometry (GC-MS) yielded a strong peak assigned to 18 O 2 , and the peak intensity increased with the extension of photoirradiation (Fig. 4d). Moreover, a stronger 18  On the other hand, the presence of edetate disodium (EDTA-2Na) in the reaction system exerted a significantly positive effect on the H 2 O 2 amount, which could be attributed to the suppressed charge recombination by the EDTA-2Na hole acceptor. Moreover, a significant amount of O 2 was monitored after light-irradiating the N 2 -saturated seawater containing NaIO 3 additive, implying that O 2 was exclusively generated from four-electron H 2 O oxidation ( Supplementary Fig. 33). Interestingly, a small amount of H 2 O 2 was also detected in the N 2saturated seawater ( Supplementary Fig. 34), and tert-butyl alcohol (TBA) additive (•OH scavenger) showed slightly negative influence on H 2 O 2 amount, verifying that the partial H 2 O 2 can be produced via the stepwise one-electron water oxidation pathway with •OH as the reactive intermediate. Isotope experiments also exhibited that 18 O 2 was detected when H 2 18 O was used after 2 h photoirradiation (Fig. 4e), and increased content of 18 O 2 was also traced by adding MnO 2 into the photoproduced H 2 O 2 solution, further implying that H 2 18 O was oxidized into both 18 O 2 and H 2 18 O 2 over Co-CN@G. Furthermore, the further increase of 18 O 2 under simulated sunlight illumination also indicated that the photothermal effect could accelerate the reaction kinetics of both two-electron and four-electron water oxidation. We then performed DFT calculations to get a further fundamental understanding of the photocatalytic activity of Co-CN@G. As for H 2 O 2 production from O 2 reduction, the O 2 chemisorption should be a prerequisite step for its subsequent reduction. Based on the Mulliken population analysis, Co single atoms on CN@G hybrid created polarized active sites on which O 2 adsorption preferably occurred. Under optimized conditions, the negative adsorption energy of O 2 (−0.28 eV) on Co-CN@G was estimated (an exothermic process) (Fig. 5a), which was significantly lower than that on Co-CN (E ads = 1.15 eV) and CN@G (E ads = 1.14 eV) (Supplementary Fig. 35 1.30 Å. Such bond configurations could enhance the interaction between O 2 and multilevel melon plane tremendously 51,52 . Furthermore, notable electron accumulation on the coordinative O 2 and electron depletion on the Co sites were found from the charge density difference (Fig. 5b), confirming the occurrence of electron backdonation from Co-single-atom to the absorbed O 2 . Thus, the photogenerated electrons on the conjugated melon units of Co-CN@G can be donated to the anti-bonding orbital of O 2 through the strong electron coupling effect between O 2 and Co, boosting O 2 activation and subsequent protonation 17 . As shown in Fig. 5c, the photoexcited electrons on the conduction band (CB) can easily drive the step-wise two-electron O 2 reduction with •O 2 − as the intermediate. Furthermore, O 2 reduction towards H 2 O 2 formation was also energetically favorable over Co-CN@G (the Gibbs free energy from [O 2 + *] to OOH* was only −0.93 eV, a thermodynamic exothermic step) (Fig. 5d). In sharp contrast, the calculated energy barrier for the intermediate OOH* formation on CN@G and Co-CN was increased by 1.57 and 1.87 eV, respectively. These results suggested that Co-CN@G greatly promoted the two-electron O 2 reduction process, ensuring efficient H 2 O 2 photosynthesis. In addition, H 2 O 2 production from direct water oxidation occurred at the N atoms (NC 2 site) of the melem unit adjacent to the Co site was also examined ( Fig. 5e and Supplementary Fig. 36). The generation of the intermediate state OH* was the most critical step for water oxidation 53 . The results revealed that Co-CN@G sample exhibited the smallest ΔG for the formation of OH* compared to CN@G and Co-CN. Thus, Co-CN@G was also more active for photocatalytic H 2 O 2 production via water oxidation. Indeed, the electrochemical linear sweep voltammogram (LSV) also demonstrated a higher current for water oxidation on Co-CN@G than on other catalysts ( Supplementary  Fig. 37). These theoretical calculations implied that the marriage between Co single atoms and CN@G heterostructure favored H 2 O 2 production from water oxidation and O 2 reduction, in consistent with the experimental results.

Discussion
In summary, we reported a photothermal-photocatalytic Co-CN@G particulate photocatalyst for highly efficient non-sacrificial H 2 O 2 production from natural seawater. By taking advantage of the cooperation of CN@G heterostructure and isolated Co atoms, this system exhibited excellent catalytic activity towards H 2 O 2 photosynthesis. Experimental results also confirmed that the photothermal effect of the Co-CN@G played a positive role in strengthening the photocatalytic activity. A high SCC efficiency of 0.72% was achieved under simulated sunlight irradiation, superior to all currently reported particulate semiconductors for H 2 O 2 photosynthesis from pure water and natural seawater. Impressively, when irradiated under outdoor sunlight, the formation rate of H 2 O 2 reached 8508 μmol g −1 day −1 in the summer. Theoretical calculations revealed that the synergy between Co single atoms and CN@G heterostructure could promote the interfacial electronic coupling and lower the associated energy barriers of O 2 reduction and water oxidation, enabling Co-CN@G to exhibit excellent photocatalytic performance for H 2 O 2 synthesis. This work provides significant guidelines for the rational design of cost-effective photocatalytic systems for future large-scale H 2 O 2 photosynthesis.

Synthesis of photocatalysts
Reduced graphene oxide (RGO) foam was prepared from GO suspension (0.9 mg mL −1 , 80 mL) through a hydrothermal method in a Teflonlined stainless-steel autoclave (100 mL) at 120°C for 5 h. The resulting foam hydrogel was washed with deionized water and obtained by freeze-drying. Subsequently, the as-prepared RGO foam was soaked into an aqueous solution (30 mL) containing 10 mg of CoCl 2 ·6H 2 O and 500 mg of thiourea for 12 h to facilitate the absorption of Co-thiourea complex on RGO. Afterward, the foam was freeze-dried for 48 h and was treated at 400°C (ramp rate: 2°C min −1 ) for 2 h under an Ar atmosphere.

Photocatalytic reaction
Totally, 50 mg of the photocatalyst was ultrasonically dispersed into the O 2 -saturated seawater (50 mL) in a sealed jacketed glass reactor. Natural seawater collected from Lingshui Bay (Lingshui City, Hainan, China) was filtered using a 0.22 μm PTFE syringe filter prior to use. The photocatalytic reaction was conducted by photo-irradiating the stirring solution with 3A solar simulator equipped with different light filters. To evaluate the stability of photocatalysts, the solid samples were recovered by centrifugation and re-dispersed in the O 2 -saturated seawater for the next cycle. The simulated seawater was obtained by adding various inorganic salts (NaCl: 27.21 g; MgCl 2 : 3.81 g; MgSO 4 :1.66 g; CaSO 4 : 1.40 g; K 2 SO 4 : 0.58 g; K 2 CO 3 : 0.21 g; MgBr 2 : 0.08 g) into 1 L of ultrapure water. The formed amount of H 2 O 2 was determined according to the colorimetric N, N-diethyl-phenylenediamine (DPD) method 54 . The wavelength-dependent apparent quantum efficiency (AQE) was measured under monochromatic light illumination, with AQE determined using Eq. (4) 55 16 O in a 100 mL borosilicate glass vessel. The sealed glass flake was bubbled with Helium atmosphere for 30 min under stirring, 18 O 2 gas (~99% purity) was then bubbled into the dispersion using a plastic syringe. The 18 O 2saturated solution was irradiated with different light sources for 15 h. The resulting reaction solution was subsequently bubbled with a Helium atmosphere for 45 min to remove the residual 18 O 2 gas. Finally, 30 mL of the solution was transferred into another sealed flask containing MnO 2 and Helium gas using a plastic syringe, and the generated gas via the decomposition of the solution was monitored by a GC-MS system. The detected 16 O 2 originated from the air when the decomposed 18 O 2 gas was injected. For the triphenylphosphine (PPh 3 ) capping experiment, the filtered solution after the reaction was bubbled with N 2 for 2 h. In a glove box, 2 mL of solution was added to 20 mL of PPh 3 solution (5 mM in acetonitrile) and co-incubated in the dark for 3 h. The resulting solution was then analyzed by GC-MS system.  18 O in a 20 mL sealed glass vial. The photocatalytic reaction was performed for 2 h under different photoirradiation. The gas produced in the headspace of the sealed glass vial was detected by GC-MS system. In addition, the resulting reaction solution was also bubbled with a Helium atmosphere for 60 min in the dark to remove the O 2 gas in the solution. Then, 20 mL solution was transferred into another sealed glass vial containing MnO 2 and Helium gas by a plastic syringe, and the evolved gas was analyzed by GC-MS system 57 .

Photoelectrochemical or electrochemical measurements
Electrochemical impedance spectroscopy and the photocurrent response analysis were conducted in a three-electrode electrochemical system (CHI-660E, Pt wire counter electrode, Ag/AgCl reference electrode) under simulated solar illumination. The photocatalysts deposited on the FTO glass (1 × 2 cm in size) was utilized as working electrode, which was fabricated according to the following steps: the photocatalyst (10 mg) was fully dispersed into a mixture containing isopropanol (1 mL), and Nafion (10 μL, 5 wt%) through ultrasonic treatment to obtain a homogenous ink. Subsequently, the ink was deposited onto the cleaned FTO substrates by dip-coating with an active area of 1 cm 2 . The photoelectrodes were then dried at 50°C in a vacuum oven. Na 2 SO 4 or KCl aqueous solution (0.1 M) served as the electrolyte. Before the photo-illumination, N 2 was continuously bubbled into the electrolyte for 40 min. The deposited photocatalyst was irradiated from the back side of the FTO electrode. Rotating disk electrode analysis was conducted on a Pine Research Instrumentation in O 2 -saturated 0.1 M phosphate buffer solution (Pt counter electrode, Ag/AgCl reference electrode). The scan rate was set to be 5 mV s −1 . The electron transfer number for O 2 reduction was reckoned according to Koutecky-Levich Eq. (6) 58 : J is tested current density, J k is kinetic current density, ν is kinetic viscosity of water, F is the Faraday constant, ω is rotating speed, C 0 is O 2 concentration in water, and D 0 is the O 2 diffusion coefficient of O 2 .

Calculation details
Density functional theory calculations were realized based on the CP2K code 59 . To model the extended systems, Gaussian and Plane Wave strategies were adopted 60 . The generalized gradient approximation based on Perdew-Bruke-Ernzerhof functional and the Goedecker-Teter-Hutter pseudopotentials were utilized to deal with the exchange-correlation energy 61,62 . The periodic boundary conditions in all directions have been exerted for all model systems. The adsorption energy E ads was determined according to E ads = E AB -E A -E B (E AB is the energy of adsorbed structure, E A is the energy of adsorbate, and E B is the energy of adsorbent).

Characterization
The microstructure of the photocatalysts was investigated by a Hitachi S-4800 field emission SEM and a JEOL-2010 TEM. XRD was examined on a Shimadzu XRD-6000 diffractometer (Cu Kα, 1.54178 Å). AC-HAADF-STEM was performed on a JEM-ARM200F instrument. The atomic and electronic structures of samples were investigated by XAFS measurements in the BL14W1 station (Shanghai Synchrotron Radiation Facility). Magic-angle-spinning NMR analysis was conducted on a Bruker Avance-III spectrometer. The elemental Co amount was detected on an Optima 7300DV ICP-OES. XPS was conducted on an ESCALAB 250 spectrometer. The specific surface area was characterized by adsorption-desorption analysis at 77 K and determined according to the Brunauer-Emmett-Teller method. The absorption of photocatalysts was implemented on a Cary 300 spectrophotometer. The PL spectra were collected on a JASCO FP-6500 spectrofluorometer (excitation wavelength: 375 nm).

Data availability
The data that support the finding of this study are available from the corresponding author upon request. Source data are provided in this paper.